1. Field of the Invention
The present invention relates to an image processing apparatus and a method thereof.
2. Description of the Related Art
Inkjet printers generate print data by converting multilevel data concerning an input image into binary data by using, for example, an error diffusion method. After the print data to be printed by means of a printhead is generated, in order to consider characteristics of the printhead and suppress deviations caused by mechanism control, multipass printing is performed. In such multipass printing, data that can be printed in one scan across a recording medium is divided into a plurality of data groups and the data groups are printed in a plurality of scans across the recording medium. In the multipass printing, a mask pattern for passes is prepared in advance. By performing logical multiplication of the mask pattern and the print data, the print data can be distributed among the data groups for a plurality of scans across the recording medium (hereinafter referred to as pass distribution processing). Such a mask pattern is designed to allow all the print data to be output in a plurality of scans across the recording medium.
In the mask pattern, predetermined printable dots are assigned to each of the passes, assuming that all printable dots represent 100%. Printable dots assigned to a pass are not included in the other passes. The logical sum of printable dots of all the passes is equal to the entirety of a print area. Thus, the mask pattern is designed to be a pattern having a maximum degree of randomness. In addition, the mask pattern is designed in a manner such that the generated print data is basically equally distributed among a plurality of scans across the recording medium. This is performed in order to cause the print data obtained by performing image processing on an input image to be printed equally among the passes.
FIG. 1 is a diagram showing a pixel lattice, ink droplets, and print duties on a recording medium.
Referring to FIG. 1, broken lines represent a lattice, and circles represent ink droplets that have landed on the recording medium. Numbers on the left of the drawing represent print duties. A print duty of 100% represents a state in which ink is to be ejected onto all pixels of the lattice. Here, in FIG. 1, the ink droplets are arranged so as to help understanding of relationships between the print duties and the ink droplets, and thus the ink droplets are not always required to be arranged as shown in FIG. 1.
As shown in FIG. 1, the ink droplets are larger in size than the pixels. Each of the pixels has a rectangular shape and each of the ink droplets, which has landed on and been absorbed into the recording medium, has an approximately circular shape. If printing is performed with a print duty of 100%, the surface of the recording medium needs to be completely covered with ink droplets. Thus, the ink droplet needs to be at least as large as a circumcircle of the pixel.
In actual printing, a mechanical system including, for example, a paper feed mechanism and a printhead-moving mechanism is involved. Thus, the mechanical system may cause more than a little deviation due to mechanism control. Moreover, the printhead also may cause a deviation when ink is ejected. In order to perform stable printing despite such deviations, ink droplets need to be set to be large in size compared with pixels.
Here, even though the same amount of ink is ejected, the size of an ink droplet on a recording medium varies according to a combination of the ink and recording medium used. In general, when an inkjet printer is used, the type of ink is fixed by setting an ink tank inside the body of the inkjet printer. In accordance with the purpose of performing printing, a recording medium is selected from among normal paper and various types of dedicated recording paper. Thus, in a case in which the type of ink is fixed, the size of an ink droplet on a recording medium varies according to the type of recording medium.
The reason that the ink droplet shown in FIG. 1 is larger in size than the pixel is described above. Next, when printing is performed with ink droplets that are larger in size than pixels, the way in which the ink droplets cover a recording medium in a case in which a print duty gradually increases will be described.
If printing is performed with a print duty of 12.5% or 25% as shown in the upper part of FIG. 1, adjacent ink droplets do not overlap each other. However, if printing is performed with a print duty of 37.5%, adjacent ink droplets overlap each other, and if printing is performed with a print duty of 50%, most of a recording medium is covered with ink droplets. The proportion of the recording medium covered by ink droplets is hereinafter referred to as “coverage”.
FIG. 2 is a graph of ink-droplet coverage versus print duty. The horizontal axis represents the print duty, and the vertical axis represents the coverage. Here, FIG. 2 is a graph in a case in which the ratio between the size of a pixel and that of an ink droplet is fixed for illustrative purposes as an example, and thus this ratio does not have to be used in an actual printer.
There is a strong relationship between ink-droplet coverage and output density although the relationship is affected by the type of recording medium. Therefore, the following description will be made in terms of ink-droplet coverage instead of output density.
As shown in FIG. 2, when the print duty is 50%, the coverage greatly exceeds 90%. If the print duty is over 50%, since little space is left, no matter how much ink is ejected, the coverage does not increase. Some recording media have a thick ink-absorbing layer, allowing printing to be performed even if a print duty is over 100% on such recording media. Moreover, output density may be increased on some recording media in response to the amount of ejected ink droplets. With respect to such recording media, however, the output density for a print duty between 50% and over 100% increases slower than the output density for a print duty between 0 and 50%.
It has been mentioned that there is a strong relationship between ink-droplet coverage and output density; however, with respect to output density, the maximum density is determined on the basis of the amount of ink absorbed into a recording medium. Some types of recording paper dedicated to inkjet printers have a coating layer that can absorb a lot of ink, the coating layer being provided on a surface of the recording paper. On such recording paper, output density increases even if a print duty is over 100%. Depending on the type of recording paper, output characteristics change on the basis of the amount of ink absorbed into a recording medium, the spread of ink, or the like.
FIG. 3 is a graph showing a relationship between density of an input image (hereinafter referred to as input density) and output density on a recording medium. As shown in FIG. 3, the relationship between input density and output density is illustrated with not a linear line but a curved line, having a convex shape in the upward direction.
As described above, in pass distribution processing, generated print data is distributed among, for example, four passes by performing logical multiplication of a mask pattern whose pattern is random and the generated print data. Thus, the generated print data is equally distributed among four data groups. That is, this means that if printing is performed with a print duty of 100%, input density is divided into four ranges k1 through k4 shown in FIG. 3 in a manner such that the ranges k1 though k4 have the following relationship,k1:k2:k3:k4=1:1:1:1.
FIGS. 4A though 4D are diagrams used to describe multipass printing in which an image is formed on a recording medium by causing a printhead to scan across a recording medium a plurality of times. Here, operations performed in the case of four-pass printing will be described.
A plurality of nozzles are arranged on a printhead 300 along a paper feed direction. A nozzle area 300a indicates an area including the first (the lowest) one-fourth of the nozzles of the printhead 300. Similarly, a nozzle area 300b indicates an area including the second (the second from the lowest) one-fourth of the nozzles of the printhead 300. A nozzle area 300c indicates an area including the third (the third from the lowest) one-fourth of the nozzles of the printhead 300. A nozzle area 300d indicates an area including the last (the highest) one-fourth of the nozzles of the printhead 300.
Printing is repeatedly performed by feeding a recording medium 310 using a paper feed mechanism after the printhead 300 has scanned across the recording medium 310 in a main scanning direction. Referring to FIGS. 4A through 4D, the recording medium 310 is moved upward from the printhead 300, and the printing is repeatedly performed.
FIGS. 4A through 4D illustrate a position of the printhead 300 relative to the recording medium 310.
FIG. 4A illustrates the first scan performed to print a first area 310_1 that is a predetermined area of the recording medium 310. The first area 310_1 corresponds to the nozzle area 300a for a first pass. In the first scan, print data for the first pass among print data for the first area 310_1 is supplied to nozzles included in the nozzle area 300a, and the printhead 300 scans across the recording medium 310 from the right to the left (or from the left to the right). In this way, printing for the first pass is performed in the first area 310_1.
In the first scan, print data is not supplied to nozzles in the nozzle areas 300b through 300d and thus printing is not performed by the nozzles in the nozzle areas 300b through 300d. After the first scan is complete, the recording medium 310 is fed by about one fourth the length of the printhead 300, that is, the length of the nozzle area 300a. 
FIG. 4B illustrates the second scan. In FIG. 4B, a current position of the printhead 300 relative to the recording medium 310 is shown, and a printhead 300_1 represented by a broken line indicates the position of the printhead 300 relative to the recording medium 310 in the first scan.
In the second scan, print data for the first pass among print data for a second area 310_2 is supplied to the nozzles included in the nozzle area 300a. At the same time, print data for a second pass among the print data for the first area 310_1 is supplied to the nozzles included in the nozzle area 300b. The printhead 300 scans across the recording medium 310 from the left to the right (or from the right to the left). In this way, printing for the second pass is performed in the first area 310_1, and printing for the first pass in the second area 310_2 is performed.
In the second scan, print data is not supplied to the nozzles in the nozzle areas 300c and 300d since the nozzles have not reached a printing area, and thus printing is not performed by the nozzles in the nozzle areas 300c and 300d. After the second scan is complete, the recording medium 310 is fed by about one fourth the length of the printhead 300, that is, the length of the nozzle area 300a. 
FIG. 4C illustrates the third scan. In FIG. 4C, a current position of the printhead 300 relative to the recording medium 310 is shown. The printhead 300_1 represented by a broken line indicates the position of the printhead 300 relative to the recording medium 310 in the second scan and a printhead 300_2 represented by a broken line indicates the position of the printhead 300 relative to the recording medium 310 in the first scan.
In the third scan, print data for the first pass among print data for a third area 310_3 is supplied to the nozzles included in the nozzle area 300a. At the same time, print data for the second pass among the print data for the second area 310_2 is supplied to the nozzles included in the nozzle area 300b. At the same time, print data for a third pass among the print data for the first area 310_1 is supplied to the nozzles included in the nozzle area 300c. The printhead 300 scans across the recording medium 310 from the right to the left (or from the left to the right). In this way, printing for the third pass is performed in the first area 310_1, printing for the second pass is performed in the second area 310_2, and printing for the first pass is performed in the third area 310_3.
In the third scan, print data is not supplied to the nozzles in the nozzle area 300d since the nozzles have not reached the printing area, and thus printing is not performed by the nozzles in the nozzle area 300d. After the third scan is complete, the recording medium 310 is fed by about one fourth the length of the printhead 300, that is, the length of the nozzle area 300a. 
FIG. 4D illustrates the fourth scan. In FIG. 4D, a current position of the printhead 300 relative to the recording medium 310 is shown. The printhead 300_1 represented by a broken line indicates the position of the printhead 300 relative to the recording medium 310 in the third scan, the printhead 300_2 represented by a broken line indicates the position of the printhead 300 relative to the recording medium 310 in the second scan, and a printhead 300_3 represented by a broken line indicates the position of the printhead 300 relative to the recording medium 310 in the first scan.
In the fourth scan, print data for the first pass among print data for a fourth area 310_4 is supplied to the nozzles included in the nozzle area 300a. At the same time, print data for the second pass among the print data for the third area 310_3 is supplied to the nozzles included in the nozzle area 300b. At the same time, print data for the third pass among the print data for the second area 310_2 is supplied to the nozzles included in the nozzle area 300c. At the same time, print data for a fourth pass among the print data for the first area 310_1 is supplied to the nozzles included in the nozzle area 300d. The printhead 300 scans across the recording medium 310 from the left to the right (or from the right to the left). In this way, printing for the fourth pass is performed in the first area 310_1, printing for the third pass is performed in the second area 310_2, printing for the second pass is performed in the third area 310_3, and printing for the first pass is performed in the fourth area 310_4.
After the fourth scan is complete, the printing for the first pass performed by the nozzles provided in the nozzle area 300a through the printing for the fourth pass performed by the nozzles provided in the nozzle area 300d are executed in the first area 310_1, and thus image forming for the first area 310_1 is complete.
In this way, when the printhead scans across the recording medium a plurality of times, groups of nozzles, which are different from one another, scan across each of the areas of the recording medium using the print data for the passes, the print data being obtained from the entirety of print data in a distributed manner. This enables, compared with single-pass printing, inconsistencies in the form of streaks generated by deviations due to a paper feed mechanism to be suppressed, and degradation of image quality due to variations in nozzle characteristics (variations in the amount of ejected ink, misdirection of ejected ink droplets, or the like) to be reduced.
In a recording apparatus such as an inkjet printer, the size of a dot formed with an ink droplet and a position where the dot is formed vary due to, for example, variations in the amount of ejected ink and variations in the direction (misdirection) of ejected ink, and thus inconsistencies in density occur in a printed image. In particular, in a serial-type recording apparatus that causes a recording head (a printhead) to scan in a direction different from the array direction of recording elements (nozzles) (direction in which recording elements are arrayed), for example, in a direction orthogonal to the array direction, inconsistencies in density due to the variations described above occur as inconsistencies in the form of horizontal streaks. Thus, the inconsistencies in density tend to be visually noticeable, and cause the quality of a printed image to be lowered.
In general, in inkjet printers, image forming is performed with consideration of, for example, misplacement of dots (landed-dot shift) due to, for example, the above-described misdirection and deviations due to paper feed, and variations in the amount of ejected ink. That is, the image forming is performed using dots smaller than the size represented by recording resolution of image data, and this prevents degradation in image quality due to inconsistencies in the form of streaks and inconsistencies in density.
In order to correct the inconsistencies in density, a method in which one line of dot pattern obtained after halftoning is formed with ink ejected from a plurality of different nozzles has been proposed. This can be achieved by, for example, feeding paper by a length less than the length of the printhead and completing printing of the one line in a plurality of scans (or passes). This method is generally referred to as multipass printing or a multipass recording method, and details thereof has been described above.
As described above, print data is distributed among the passes in a manner such that almost equal numbers of dots are assigned to the passes in pass distribution processing. Thus, as shown in FIG. 3, printing, in which inconsistencies in density are included, performed in the first pass greatly affects output density.
In order to solve this problem, Japanese Unexamined Patent Application Publication No. 2004-209943 discloses a method for appropriately controlling a recording ratio of dots for each pass in response to a density level. In this method, if a density is lower than or equal to a predetermined value, the passes are designed to have an equal recording ratio. If the density is higher than the predetermined value, a recording ratio for the first pass is designed to be reduced, and a recording ratio for the second pass is designed to be increased instead. A recording ratio is controlled using a combination of a threshold table and a mask table. Since a threshold table is utilized, dithering is used as halftoning.
That is, a threshold matrix used for halftoning and a mask pattern used for multipass recording are related to each other. With respect to a group of thresholds included in the threshold matrix, a mask off ratio (for nozzles) for each of scans is controlled to perform appropriate assignment of dots (for the nozzles) for the scan in response to input density.
However, if halftoning is performed by dithering, periodic characteristics due to dithering appear in an output image. Moreover, since a combination of a threshold table and a mask table is used, there is a problem that a determined recording ratio and a dot generation ratio do not always match for some input image data. In addition, it is difficult to generate an arbitrary recording ratio, and it is also difficult to change the threshold table and the mask table in real time.
Furthermore, in general, the size of pixels and the size of ink droplets ejected from the recording head are not equal. By considering, for example, deviations due to mechanism control and recording-head characteristics, as shown in FIG. 1, ink droplets are set to be larger than pixels. Thus, the relationship between the number of ink droplets ejected per unit area (input density) and output density on a recording medium is not linear as shown in FIG. 3.
In multipass printing, if print data is equally distributed among the passes, printing performed in the first pass greatly affects output density and, as a result, printing performed in the second and later passes affects the output density to a lesser degree. For example, in a case in which printing is performed with an input density of 100% in four passes, if the input density is equally distributed among the passes so that they have 25% each, the first 25% of dots are printed in the first pass, and the next 25% of dots are printed in the second pass. Thus, 50% of dots should be printed in the first and second passes. However, as shown in FIG. 2, the coverage of ink droplets has already become over 90% on the recording medium.
In multipass printing, degradation in image quality (inconsistencies in density) due to the errors can be made less noticeable by diffusing various errors (deviations due to a paper feed mechanism, variations in nozzles of the recording head, and the like). However, printing performed in the passes does not equally affect the output density. The printing performed in the first pass most greatly affects the output density. In other words, the errors are not equally diffused in multipass printing.
FIG. 15 is a diagram showing a case in which an image is recorded in two passes.
A recording head 601 includes nozzles for N dots arranged along a sub-scanning direction. A predetermined area 603 is an area in which dots are recorded in the first pass. A predetermined area 604 is an area in which dots are recorded in the second pass. After a scan performed using the recording head 601 is complete in a main scanning direction, a recording medium 602 is fed in the sub-scanning direction by a length corresponding to N/2 dots of the recording head 601, and the recording head 601 can scan again in the main scanning direction. An image is completely printed by repeatedly causing the recording head 601 to scan and feeding the recording medium 602. That is, with respect to a certain area, an image recorded in the second pass is superimposed on an image recorded in the first pass, and the images are combined.
As described above, in the case in which the image is recorded in two passes, even if 50% of dots are formed in the first pass, the coverage of ink droplets greatly exceeds 50% and most of the paper is covered with ink droplets.
In addition, if the recording head 601 performs printing in forward and backward scans along the main scanning direction, the first color used in the forward scan becomes dominant during the forward scan and the first color used in the backward scan becomes dominant during the backward scan. Thus, if the first coloring materials used in the forward and backward scans are different, different color bands are alternately formed for every paper-feed length, and image quality is greatly reduced. This is a type of inconsistency in color. If the amount of ink per unit area ejected from the recording head 601 (a print duty) is increased, the color bands become more noticeable and severe inconsistencies in color can be recognized in an area in which printing is performed using colors with a high print duty. The inconsistencies in color may appear differently according to ink-absorbing characteristics of recording medium.